Cold cathode lamps are used as a backlight in liquid crystal displays (LCDs), as employed in computer screens for example. Similar backlights are also found in other types of displays in a wide range of applications, such as in motor vehicles, illuminated advertising panels and suchlike.
Cold cathode lamps are generally employed in backlights for LCD screens. They have the advantage of generating a small amount of heat combined with a relatively long useful life and high efficiency. Moreover, the electrode structures are simple making it possible to produce very small cold cathode lamps that can also be used in small liquid crystal displays.
A cold cathode lamp comprises a tube having a high-voltage terminal at a first end of the tube and a low-voltage terminal at the second end of the tube. The high-voltage terminal is supplied with a high-frequency AC voltage, a typical supply voltage having a frequency of approximately 50 to 100 kHz and a voltage amplitude of approximately 500 to 1000 V. The low-voltage terminal is generally connected to ground. However, it is also possible to connect the two cold cathode lamp terminals to a positive and a negative AC voltage, a virtual ground being located at about the center of the tube. This is especially practical for particularly long tubes.
A key criterion for LCDs is to illuminate the entire display surface as uniformly as possible. Depending on the size of the screen, from two to 16, or even more, cold cathode lamps are used for the backlight. The lamps are arranged parallel to each other, vertically above one another and their light is distributed on a liquid crystal plate via a reflector and via a diffuser plate. To achieve the most uniform distribution of brightness that is possible, it is not only necessary for the individual lamps to glow with the same brightness, but each individual lamp in itself must also emit a uniformly bright light along its length. An uneven distribution of brightness over individual lamps is caused by manufacturing tolerances and can be kept under control by selection during the manufacturing process. The causes of uneven brightness over the length of an individual lamp are explained below.
Cold cathode lamps in liquid crystal displays are supplied with a high-frequency AC voltage via an inverter, called a backlight inverter. A reflector directs the light emitted by the lamps onto a diffuser plate which guides and distributes it onto a liquid crystal plate. The liquid crystal plate is generally inserted between two polarization plates. The entire arrangement is held in a frame. The mechanical arrangement of the backlight inverter and the lamps in the liquid crystal display give rise to parasitic capacitances between the fluorescent tube and ground which results in the effective lamp current decreasing from the high-voltage terminal to the low-voltage terminal. This can result in a brightness that diminishes from the high-voltage terminal to the low-voltage terminal. This problem is amplified in the event that the brightness of the fluorescent lamp is lowered by analogue dimming. The lamp current can then drop in the region of the low-voltage terminal to such an extent that the lamp does not emit any light whatsoever in this region. In practice, this means that the parasitic capacitances also limit the useful analogue dimming range.
U.S. Pat. No. 6,670,781 relates to a control circuit for cold cathode lamps for LCDs and deals with the problem that, particularly for analogue dimming, these lamps emit a non-uniform brightness and flicker. To solve this problem, U.S. Pat. No. 6,670,781 proposes a new control method for fluorescent lamps that uses a predetermined number of current pulses. However, U.S. Pat. No. 6,670,781 does not deal with the problem of diminishing brightness along the length of a fluorescent lamp due to parasitic capacitances.
Other fluorescent lamps and particularly cold cathode lamps for liquid crystal displays and associated control devices are described, for example, in U.S. Pat. Nos. 6,538,373 and 6,108,215, just to mention a couple of examples.
It is the object of the invention to provide a lighting arrangement that has a fluorescent lamp and particularly a cold cathode lamp which generates a uniform brightness over its entire length in both normal operation as well as over a wide dimming range.